From da0d651197dee82dda267c84e85d1f2b89318804 Mon Sep 17 00:00:00 2001
From: Sun Yimin <emmansun@users.noreply.github.com>
Date: Mon, 26 May 2025 15:10:48 +0800
Subject: [PATCH] sm2: provide SignMessage method to comply with the
 [crypto.MessageSigner] interface

---
 sm2/sm2_dsa.go      | 11 +++++++++++
 sm2/sm2_dsa_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/sm2/sm2_dsa.go b/sm2/sm2_dsa.go
index 614b6f2..a400b06 100644
--- a/sm2/sm2_dsa.go
+++ b/sm2/sm2_dsa.go
@@ -118,6 +118,17 @@ func (priv *PrivateKey) SignWithSM2(rand io.Reader, uid, msg []byte) ([]byte, er
 	return priv.Sign(rand, msg, NewSM2SignerOption(true, uid))
 }
 
+// SignMessage signs a message with the private key, reading randomness from rand.
+// If opts is an instance of SM2SignerOption, it will use the UID from opts.
+// This method is used to comply with the [crypto.MessageSigner] interface.
+func (priv *PrivateKey) SignMessage(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error) {
+	var uid []byte
+	if sm2Opts, ok := opts.(*SM2SignerOption); ok {
+		uid = sm2Opts.uid
+	}
+	return priv.SignWithSM2(rand, uid, msg)
+}
+
 // GenerateKey generates a new SM2 private key.
 //
 // Most applications should use [crypto/rand.Reader] as rand. Note that the
diff --git a/sm2/sm2_dsa_test.go b/sm2/sm2_dsa_test.go
index 50a85f3..cdf2a5b 100644
--- a/sm2/sm2_dsa_test.go
+++ b/sm2/sm2_dsa_test.go
@@ -424,6 +424,39 @@ func TestSignVerify(t *testing.T) {
 	}
 }
 
+func TestSignMessage(t *testing.T) {
+	priv, _ := GenerateKey(rand.Reader)
+	tests := []struct {
+		name      string
+		plainText string
+	}{
+		// TODO: Add test cases.
+		{"less than 32", "encryption standard"},
+		{"equals 32", "encryption standard encryption "},
+		{"long than 32", "encryption standard encryption standard"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			signature, err := priv.SignMessage(rand.Reader, []byte(tt.plainText), nil)
+			if err != nil {
+				t.Fatalf("SignMessage failed %v", err)
+			}
+			result := VerifyASN1WithSM2(&priv.PublicKey, nil, []byte(tt.plainText), signature)
+			if !result {
+				t.Fatal("verify failed")
+			}
+			signature, err = priv.SignMessage(rand.Reader, []byte(tt.plainText), NewSM2SignerOption(true, []byte("testid")))
+			if err != nil {
+				t.Fatalf("SignMessage failed %v", err)
+			}
+			result = VerifyASN1WithSM2(&priv.PublicKey, []byte("testid"), []byte(tt.plainText), signature)
+			if !result {
+				t.Fatal("verify failed")
+			}
+		})
+	}
+}
+
 func TestSM2Hasher(t *testing.T) {
 	tobeHashed := []byte("hello world")
 	keypoints, _ := hex.DecodeString("048356e642a40ebd18d29ba3532fbd9f3bbee8f027c3f6f39a5ba2f870369f9988981f5efe55d1c5cdf6c0ef2b070847a14f7fdf4272a8df09c442f3058af94ba1")